The present invention relates to a tapping screw.
A conventional tapping screw has a thread with generally a uniform lead angle on the periphery of its shank over its entire length. In order to drive in the screw, it has to be turned while pressing very hard from the start. Further, it takes much time before the screw is driven in deep enough for it to stand on its own. Only a little relaxing of the driving force may cause the screw to skid, making it impossible to further drive in the screw.
In order to solve the abovesaid problems, the present applicant proposed a tapping screw (Japanese Unexamined Utility Model Publication No. 62-16992) in which the lead angle of its thread 10 increases toward the tip of the tapered portion 11, as shown in FIG. 4. With this prior art tapping screw, a thread 10' on the tip of the shank has such a lead as to be convergent to the axis of the screw so that the thread 10' will act as a drill edge, thus considerably improving the drilling capacity of the screw and permitting a positive driving.
With the conventional drill, the tip of its tapered end is adapted to be pressed against a clamp face to bite into it. If the clamp face is made of hard wood or the like, a great deal of force is needed for initial driving of the screw, and also much time is required for the screw to stand on its own. After a bore has been drilled by the tapered tip portion, the thread on the large-diameter shank bites into the bore with its leading end first, when a large torque is required to turn the screw and the thread is liable to slip and idle against the hard clamp face, inhibiting further driving of the screw.